The primary goal of this study is to demonstrate that calcium (Ca) uptake, transport and deposition in a marine bivalve mollusc (Mercenaria mercenaria) is an excellent invertebrate model for Ca regulation in the mammalian system. Although many of the same processes are already known to be present in both the invertebrate and vertebrate systems, additional similarities will be investigated to determine whether (1) Ca uptake/release by bivalve mantle cells is similar to mammalian ileal mucosal cells (i.e. mediated by active transport); (2) the distribution of Ca in bivalve blood (% free Ca2+; % protein-bound Ca) is similar to that of mammalian blood; (3) a specific Ca-binding protein is present in bivalve mantle cells; and (4) mammalian regulators of Ca metabolism (parathormone, calcitonin, vitamin D2, 1,25 dihydroxycholecalciferol) effect Ca uptake/regulation in the bivalve. Experiments will utilize whole bivalves, dissociated mantle cells (inner and outer mantle epithelia separately), and bivalve blood plasma. The radiotracer Ca45 will be used to follow Ca uptake, storage and transfer in clam tissues, subcellular fractions and plasma. Protein characterizations will be conducted by polyacrylamide gel electrophoresis (both native and SDS PAGE), protein electroblotting (PVDF membranes) and autoradiography. The bivalve model for Ca regulation promises to be a convenient and easily manipulated system for studies on hyper- and hypocalcemia, the regulation of Ca deposition, and perturbations of normal calcification processes.